1. Field of the Invention
The present invention relates to a method for making a design and/or sign on each of a number of glass, glass-ceramic and ceramic articles using a transfer agent. The invention also relates to an apparatus for performing the method according to the invention, i.e. to an apparatus for making a design and/or sign on each of a number of glass, glass-ceramic and ceramic articles.
2. Prior Art
Designs are extensively used on glass, glass-ceramic and ceramic products to provide desired esthetically pleasing impressions. Lettering, signs or the like must be applied to certain products, in order to e.g. provide required information directly to the user. Glass-ceramic panels for cooking ranges, which have other operation and status information, and also colored designs, in addition to the trademark, e.g. CERAN.RTM., should be mentioned here as a typical example.
Different methods are known for making these designs and/or signs, of which two are currently of practical importance.
In the first typical process ceramic pigments are directly printed on the glass and ceramic products by means of relevant printing techniques, the pigments are first dried so that they cannot be smudged and after that they are baked on. In the case of the above-mentioned glass-ceramic panel for the cooking range the pigments are typically baked on during the ceramicizing. In this case the printing does not occur on the manufactured glass-ceramic panel, but already on the blank to be ceramicized.
A permanent printed image is obtained on the glass or ceramic product by the above-mentioned process.
A screen stencil must first be prepared in the screen printing process used in general for the above-described printing process. Also the screen is made from a fine-meshed textile or wire web, which is held taught by means of a screen frame, which is covered on its image-free regions or positions with cut paper pieces, with inks for producing the signs or with a photographically made stencil. The ceramic pigments are applied directly to the glass or ceramic product by means of this screen stencil.
The making of the screen stencil is very expensive with known methods and unproductive for individual products. Also the printing pigments are applied through the open regions of the screen stencil manually with the help of a wiper or in a screen printing machine. The screen printing process is a wet process, in which a paste of ceramic colored pigments with printing oils is used as a colored ink, so that comparatively large and expensive machines with dryers are required. Also large work protecting and environmental problems are required in regard to the manufacturing process. The solvents present in the printing oils are comparatively highly volatile, so that expensive and troublesome work protecting measures must be taken and separate filter units are required. Besides in screen printing methods, as in offset printing, several printing steps are required in succession for different colors (e.g. cyan, magenta, yellow and black). This leads to a very large plant.
Furthermore in the known printing methods the reproducibility of colors for a comparatively large number of pieces cannot be guaranteed and also already after a comparatively small number of pieces, i.e. after about 100 printing processes, the screen stencil must be cleaned.
Among other disadvantages the resolution of the printed colors is limited by the grid of the screen stencil in screen printing processes. This means that the printed ceramic or glass products are frequently not satisfactory in regard to smoothness, uniformity and resolution of the color printing. Frequently in order to produce a desired product several special pigments must be used.
In the second type of process ceramic pigments are not applied directly on the glass and ceramic article, but instead on a transfer agent, such as a paper coated with gum arabic. The transfer agent prepared in this way is applied to the ceramic or glass article at the desired position and moistened, so that the paper may be removed leaving the pigments on the article (pressed image). Also transference by heating is also known. Finally the product is baked in a known manner, which leads to fusion of the ceramic pigment with the product. A permanent printed image is thus produced on the ceramic or glass article in this manner.
This second type of process operates according to the principle of image transference.
It is thus known to apply the ceramic pigments to the transfer agent by means of appropriate printing techniques, especially by means of screen printing, with the above-mentioned disadvantages of the screen printing techniques.
DE 44 13 168 C2 discloses a new type of ceramic toner, i.e. ceramic pigment composition for use instead of the conventional ceramic printing pigments or inks. This ceramic toner comprises fine particles of ceramic pigments, flux (glass), binding agent resin or resins and standard additives. It may be applied by means of an electrophotographic reproduction method (electro-copying process) to the transfer agent.
Using these features it is possible to provide a method of making decorated ceramic and glass articles that overcomes the above-described problems of the printing methods previously known in the art. A decorated ceramic and glass article can be produced, which is superior to the previously known decorated article (made by printing) in regard to fineness and resolution.
The known process permits a simple application of the image to the transfer agent.
Data for the desired images to be applied to the ceramic or glass article may be read in, e.g. by means of a digital color scanner or by using original graphic data. This data may then be transferred by means of a personal computer into an image memory of an electrophotographic reproduction device, e.g. a laser printer. This makes changes advantageously possible without much additional effort or expense, for example in the pigment shades, the resolution. Also graphic art and/or modifications can be transferred directly to the transfer agent by means of modern computer technology.
A disadvantage of this known process is that the images are printed on discrete sheets of the transfer agent, which must be handled in connection with further processing steps, in order to apply the images from these discrete sheets to the glass or ceramic article. Thus multiple steps are required for making the images on the article. In addition, because of the required individual handling of the discrete sheets and the exact application of the image on the glass or ceramic article, there is a danger that the images slip as they are transferred from the moistened or heated transfer agent during application. This behavior has been long known for transferred images. The images then are often distorted or deformed during application, and the tolerance requirements for the colored product are not satisfied.